This invention relates to a drying screen and basket for a centrifugal dryer used to remove water and fines from an aqueous slurry of pulverized coal.
Pulverized coal slurry centrifugal driers may include a frustoconical screen, supported by a basket of similar geometry, both supported for rotation about a vertical axis, and downwardly divergent. The screen/basket assembly is rapidly rotated (e.g., at 600-900 rpm), and slurry is introduced into the top end of the assembly. The slurry strikes the screen at its top interior, and tends to move downward to the larger diameter portion of the screen as the slurry is centrifuged. Water and fines smaller than the screen openings escape through the screen, while larger particles are harvested by a more slowly rotating (e.g., 75 rpm) tapered auger blade which plows these larger particles downward to an outfeed conveyor. The fines and water are directed to a separate trough, from which they are removed as waste. The clearance between the auger blade and the screen interior is on the order of 0.010-0.015 inches.
Drier screen, being of fine mesh, are susceptible to abrasive wear in service, and in fact, these screens must be replaced with great frequency. Any replacement is attended by considerable downtime, and not inconsiderable expense. Presently, the entire screen must be replaced when it is worn, despite the fact that the wear is almost exclusively confined to the point of the screen which bears the initial impact of the slurry, that is, the top few inches of the screen.
It is therefore an object of the invention to provide the industry with a drying basket having increased durability, yet having lower replacement cost. This object is achieved by providing a centrifugal drying basket/screen assembly formed in two sections: a lower major section, and a smaller upper section, so that only the upper section need be regularly replaced. The lower portion of the assembly can be replaced only as needed, on a much more infrequent basis.
A further object of the invention is to reduce the effect of slurry impact at the upper portion of a two-part screen/basket assembly, and thus prolong the life of even the upper portion. This objective is satisfied by creating a circumferential ridge on the interior surface of the drying screen, just below the point of initial slurry impact. The ridge acts as a dam, preventing particles from passing downward along the screen at that point, so that a cake of particles are built up inside the screen at that point. This cake then receives direct impact from entering particles, thus protecting the underlying screen.
A further object of the invention is to provide a centrifugal coal dryer with a basket/screen assembly having integral fins to create a vacuum to assist in removing water from the slurry.
Another object of the invention is to provide a two-part basket/screen assembly that is easily disassembled and rebuilt. Other objects of the invention will be apparent from the following description.